Monthly Northern Hemisphere Annular Mode (NAM) Index

or Arctic Oscillation (AO) Index

The monthly NAM index (NAMI) or AO index (AOI) is defined as the difference in the normalized monthly zonal-mean sea level pressure (SLP) between 35°N and 65°N (Li and Wang, 2003). The NAMI is one measure of the hemispheric-wide  fluctuations in surface air pressure occurring between the mid- and high-latitude Annular Belt of Actions (ABA). At the positive NAM phase the Northern Hemisphere (NH) winter circulations are characterized by: low-pressure anomalies throughout the polar region, high-pressure anomalies across the subtropical and mid-latitudes, stronger-than-normal surface westerlies across midlatitudes, weakened subtropical jet and strengthened polar jet, positive temperature anomalies over midlatitudes in troposphere, negative temperature anomalies over polar cap in the upper troposphere and lower stratosphere, increased Ferrel Cell (FC) with a maximum anomalous ascent near 65°N and a minimum anomalous descent between 35-40°N reduced polar cell and splitting Hadley Cell into two parts, enhanced south wind over subtropical and mid-latitudes, baroclinity enhances in higher latitudes and weakens in subtropical zone, etc. The daily NAM index (NAMI) or AO index (AOI) is defined as same as the monthly index except for the daily resolution.

Correlation maps between monthly SLP anomalies in the NH (1958.1-2000.12) and (a) the NAMI (Li and Wang, 2003) and (b) the AO index (Thompson and Wallace, 1998). The contour interval is 0.1. The red (blue) shading indicates significant at the 99.9% confidence level.

The NAM index. Top. DJF (1872-2010); Middle. JJA (1873-2010); Bottom. Annual (1873-2010)

The yellow lines indicate 7-year Gaussian-type filtered values.

Schematic of the NH normal winter circulations and the anomalous winter circulations at positive NAM phase (Note that the part of the image shown below the abscissa down to the curved boundary at the bottom is the pictorial of horizontal motions and pressure anomalies at the earth surface).

Related References

• Li, J. P., and J. X. L. Wang, 2003: A modified zonal index and its physical sense. Geophys. Res. Lett., 30(12), 1632, doi:10.1029/2003GL017441. PDF 
• Li, J. P., 2005: Coupled air-sea oscillations and climate variations in China, in: Climate and Environmental Evolution in China (First Volume), Edt. D. Qin, Beijing: China Meteorological Press, 324-333. PDF
• Thompson, D. W. J, and J. M. Wallace, 1998: The Arctic Oscillation signature in the wintertime geopotential height and temperature fields. Geophys. Res. Lett., 25, 1297-1300. PDF 

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